Accelerometers have found real-time applications in controlling and monitoring military and aerospace systems. For example, the basis of many modern inertial guidance systems is an arrangement that comprises three mutually perpendicular accelerometers, which can measure forces in any direction in space, coupled with three gyroscopes, also with mutually perpendicular axes, which constitute an independent frame of reference. An accelerometer measures acceleration or, more particularly, the rate at which the velocity of an object is changing. Because acceleration cannot be measured directly, an accelerometer measures the force exerted by restraints that are placed on a reference mass to hold its position fixed in an accelerating body (such as, for example, a suspended mass secured by springs within a housing). As is appreciated by those skilled in the art, acceleration is generally computed using the relationship between restraint force and acceleration given by Newton's second law: force=mass×acceleration.
The output of an accelerometer is generally in the form of a varying electrical voltage. As an object (attached to an accelerometer) accelerates, inertia causes the reference to lag behind as its housing moves ahead (accelerates with the object). The displacement of the suspended mass within its housing is proportional to the acceleration of the object. This displacement may be converted to an electrical output signal by a pointer (fixed to the mass), for example, moving over the surface of a potentiometer. Because the current supplied to the potentiometer remains constant, the movement of the pointer causes the output voltage to vary directly with the acceleration.
Specially designed accelerometers have been used in applications as varied as control of industrial vibration test equipment, detection of earthquakes (seismographs), and input to navigational an inertial guidance systems. The design differences are, primarily concerned with the method used to convert an accelerometer's output signal to an appropriate acceleration reading. In this regard an accelerometer's output may have two components: an output signal that is proportional to the force exerted by Earth's gravity at or near the surface of the earth (i.e., static acceleration), and another output signal that is proportional to the force exerted by shocks or vibrations (i.e., dynamic acceleration). Depending on the application, a signal-conditioning circuit may be required. With the advent of microelectromechanical systems (MEMS) technologies, the size and costs of accelerometers have been greatly reduced.
Recently, accelerometers have been used to detect the amount of time spent off the ground by a person during a sporting movement such as, for example, skiing, snowboarding, and biking. Exemplary in this regard are the devices disclosed in U.S. Pat. No. 5,636,146, U.S. Pat. No. 5,960,380, U.S. Pat. No. 6,496,787, U.S. Pat. No. 6,499,000, and U.S. Pat. No. 6,516,284. All of these closely related patent documents disclose, among other things, accelerometer-based apparatuses that are configured to sense vibrations (i.e., dynamic acceleration), particularly the vibrations experienced by a ski, snowboard, and/or bike that moves along a surface (e.g., a ski slope or mountain bike trial). In these systems, the voltage output signal from the accelerometer(s) provides a vibrational spectrum over time, and the amount of hang-time is ascertained by performing calculations on that spectrum. In particular, the vibrational spectrum sensed by these prior art devices are generally highly erratic and random, corresponding to the randomness of the surface underneath the ski, snowboard, and/or bike (as the case may be). During the period of time when the ski, snowboard, or bike is off the surface (i.e., during a “hang-time” event), however, the vibrational spectrum becomes relatively smooth because there are no longer any underlying vibrations impacting on the accelerometer(s). A microprocessor subsystem is then used to evaluate the vibrational spectrum and determine the approximate hang-time from the duration of the relatively smooth portion sandwiched between two highly erratic and random vibrational spectrum portions. Because the condition of standing still (i.e., little or no movement) also results in a relatively smooth vibrational spectrum, these prior art devices require complicated timing methods to ensure that accurate results are displayed. In other words, the prior art devices have difficulty in accurately distinguishing between the conditions of standing still and experiencing hang-time.
Accordingly, there is still a need in the art for new and improved mechanisms for determining the time-of-flight or hang-time of a moving and jumping object such as, for example, a skier, snowboarder, skater, biker, or jumper. There is also a need for detecting changes in the static acceleration profile of objects for other purposes, as specified below. The presently disclosed subject matter fulfills these needs and provides for further related advantages.